In this work two induced, third order nonlinear effects were demonstrated. Both phenomena were induced on weak probe beams by a pumping beam in the resonant regime of the nonlinear materials. Both demonstrations were phase independent and were accomplished with low power considering the short interaction lengths. ^ The first phenomena demonstrated was a reduction in a monochromatic probe signal due to a pump signal. The pump and probe were perpendicularly propagated and intersect within a three millimeter square of amorphous zinc selenide. The reduction effect written onto the probe follows a nonlinear profile which saturates with a five percent reduction of the probe signal, while the pump maintains a linear absorption profile. The saturation of the induced effect occurs at low intensity of less than 12 W cm−2. For a helium neon probe, at 632.8 nanometers, the pump induced change in the index of refraction, Δn, and the induced change in the absorption coefficient, Δα, were analyzed in the saturation region using Fresnel equations and found to have the values of Δn = 0.0049 and Δα = 0.0038 mm −1. While resonant saturation effects have been seen in semiconductor doped glass before those effects required a copropagating pump that was eight orders of magnitude more intense. ^ The second portion of this work was centered on amorphous zinc selenide cascaded with semiconductor doped glass. The glass is doped with quantum dots of cadmium sulfur selenide that is available from Schott Glass. The pump used was in the resonant regime and copropagated with a white light source as a probe and both were introduced and removed from the cascaded materials by multimode fibers. The cascaded materials acted as a single period grating which preferentially allowed certain wavelengths to pass. When the material was pumped the preferential pass was reversed. The difference between the probe in the pumped and unpumped state was an expanding sinusoidal with a relative change of ±20%. The preferential pass and the pump induced shift in the preferential regions is comparable to resonance Fabry Perot etalons with an induced change in the optical path length but is considerably more robust. ^
In this work two induced, third order nonlinear effects were demonstrated. Both phenomena were induced on weak probe beams by a pumping beam in the resonant regime of the nonlinear materials. Both demonstrations were phase independent and were accomplished with low power considering the short interaction lengths. ^ The first phenomena demonstrated was a reduction in a monochromatic probe signal due to a pump signal. The pump and probe were perpendicularly propagated and intersect within a three millimeter square of amorphous zinc selenide. The reduction effect written onto the probe follows a nonlinear profile which saturates with a five percent reduction of the probe signal, while the pump maintains a linear absorption profile. The saturation of the induced effect occurs at low intensity of less than 12 W cm−2. For a helium neon probe, at 632.8 nanometers, the pump induced change in the index of refraction, Δn, and the induced change in the absorption coefficient, Δα, were analyzed in the saturation region using Fresnel equations and found to have the values of Δn = 0.0049 and Δα = 0.0038 mm −1. While resonant saturation effects have been seen in semiconductor doped glass before those effects required a copropagating pump that was eight orders of magnitude more intense. ^ The second portion of this work was centered on amorphous zinc selenide cascaded with semiconductor doped glass. The glass is doped with quantum dots of cadmium sulfur selenide that is available from Schott Glass. The pump used was in the resonant regime and copropagated with a white light source as a probe and both were introduced and removed from the cascaded materials by multimode fibers. The cascaded materials acted as a single period grating which preferentially allowed certain wavelengths to pass. When the material was pumped the preferential pass was reversed. The difference between the probe in the pumped and unpumped state was an expanding sinusoidal with a relative change of ±20%. The preferential pass and the pump induced shift in the preferential regions is comparable to resonance Fabry Perot etalons with an induced change in the optical path length but is considerably more robust. ^
I recent years, significant attention has been given to nonlinear optical phenomena, such as nonlinear optical absorption and dispersion, derived from a driven quantum transition of semiconductor nanostructures, such as semiconductor quantum wells, quantum disks and mainly quantum dots. An important issue that is widely studied is the effect of probe electromagnetic field intensity on the total (nonlinear) absorption coefficient and on the total (nonlinear) index of refraction. Some of the phenomena that are commonly found for large electromagnetic field intensities are the creation of a strong dip in the absorption spectrum near or at resonance (the so-called bleaching effect), negative absorption near or at resonance that leads to optical gain and the change of the slope of the total index of refraction near or at resonance from negative to positive that changes the behavior of the system from fast to slow light. Unfortunately, these phenomena do not exist and are artifacts of a common abuse of the theoretical methodology for the study of the total absorption coefficient and the total index of refraction. The main error is that the total absorption coefficient and the total index of refraction was initially derived from density matrix equations using perturbation theory and includes first-order (linear) as well as third-order (nonlinear) terms. This method gives correct results for small electromagnetic field intensities and it leads to saturation of absorption and dispersion in this regime. However, for larger intensities it gives the misleading results mentioned above due to either failure of the order of perturbation theory considered in the derivation of the total absorption coefficient and the total index of refraction or even failure of perturbation theory at all in the treatment of the problem. In this work, we revisit the problem of the total absorption coefficient and the total index of refraction in a symmetric semiconductor quantum dot structure (we choose the example of a core-shell quantum dot) under a strong probe field excitation. We use the two-level model, solve the relevant density matrix equations under the rotating wave approximation and under steady state conditions, and obtain the correct form of the nonlinear optical susceptibility that is then used for the derivation of the formulae of the total absorption coefficient and the total index of refraction under the interaction with a strong probe field. Actually, the commonly used perturbation theory formulae can be obtained by our formulae using a series expansion and keeping terms up to second order in electric field amplitude. We also derive the electronic structure of the quantum dot system by solving numerically the time-independent Schrodinger equation, under the effective mass approximation, using the potential morphing method and obtain the energy difference and the electric dipole matrix element for the relevant transitions of the quantum dot system. Then, for the specific quantum dot system we compare the results of the total absorption coefficient and the total index of refraction for the two methodologies (the perturbation result and our result) for different electromagnetic field intensities.
This dissertation presents investigations of fundamental optical nonlinearities in semi-conductor microstructures. Two distinct effects are studied. In the first part the excitonic optical Stark effect in InGaAs and GaAs multiple quantum-well structures is investigated by means of pump-probe spectroscopy. For nonresonant excitation below the excitonic transition the direction of the shift of the resonance depends on the polarization of the pump and probe pulses. In particular, for anti-circular polarization a surprising red-shift is observed. For resonant excitation, induced absorption energetically above and below the exciton transition and bleaching of the resonance is found. Experiments using both resonant and nonresonant excitation reveal the importance of bound and unbound two-exciton states in absorption changes of the 1s heavy-hole exciton resonance. It is found that higher-order Coulomb contributions determine the intensity as well as the time dependence of the differential excitonic absorption. In addition, the influence of light-hole excitons is analyzed. It is shown that the direction of the optical Stark shift for nonresonant excitation depends also sensitively on the heavy-hole to light-hole splitting and the detuning of the pump pulse. For very high pump intensities and nonresonant excitation the absorption is split when a circularly polarized pump and a linearly polarized probe beam are used. For co-circular excitation traces of hyper-Raman gain are observed. In the second part of this dissertation, the nonlinear optical response of semiconductor microcavities in the nonperturbative regime is studied in resonant single-beam transmission and pump-probe experiments. In both types of experiment, a pronounced third transmission peak lying spectrally between the two normal modes is observed. Its dependence on the probe intensity, pump intensity, pump-probe delay, exciton-cavity detuning and pump detuning is investigated. For single-beam transmission, the energy of the third peak parallels the position of the cavity resonance. It is more pronounced for circularly polarized excitation and lasts longer than the two normal modes. For pump-probe experiments, the third peak increases with decreasing probe intensity and increasing pump intensity. Its energy is close to the low-energy side of the pump spectrum and virtually unaffected by the cavity-exciton detuning. The appearance of the third peak requires temporal overlap of pump and probe pulses. The origin of this complex nonlinearity is the quantum nature of light, which induces intraband polarizations in the presence of a coherent driving field and a finite carrier density. It is found that the coupling of the intraband polarizations via guided modes to the polarization of the fundamental longitudinal mode is responsible for the third transmission peak. A fully quantized theory reproduces the experimental observations.
Frontiers in Guided Wave Optics and Optoelectronics 210 enable those who are not expert in the field to appreciate the subject of the research.We give a clear physical insight into the electronic and thermal RIC mechanisms, and present important estimations helping the reader to clearly understand the scientific context, motivation and aims of this work.The main body of the work is centered around two experimental studies (Fotiadi et al., 2008c; Fotiadi et al., 2009c), reproduced in parts 3 and 4. Part 3 is dedicated to the fundamental properties of the Yb-doped fibers sensitive to the diode pumping at 980 nm.In this part we show our original RIC experiments performed with different samples of aluminum-silicate fibers and report on the dynamical characteristics of the RIC effect measured in the spectral range 1460 -1620 nm.Detailed analysis of these experimental data has brought us to the conclusion that the process is predetermining by the electronic RIC mechanism that can be quantitatively described by a two-level population inversion model with the polarizability difference (PD) of the Yb-ion in the excited and ground states as the only material model parameter determined from the experimental data.The absolute PD value measured in the experiment is found to be independent on the testing wavelength and the fiber specification parameters, i.e. the cross-section profile, the size, the length and the Yb-ion concentration.Our results ensure a predominant contribution to the RIC effect from far-resonance UV electronic transitions rather than near-resonant IR transitions.Part 4 of the Chapter focuses on potential applications of the RIC effect in advanced fiber laser systems implemented through our original concept of all-fiber coherent beam combining.The concept employs Yb-doped fibers as optically controllable phase shifters to be conjugated with Er-doped fiber amplifiers operating at ~1.55 µm.The coherent combining of 500-mW Er-doped amplifiers successfully demonstrated in our experiment confirms the validity of the concept.The performance characteristics of the multichannel fiber laser system associated with the proposed power scaling method are also under discussion. RICs in Yb-doped optical fibers: the origins and fundamentalsIn this part we summarize fundamentals of the electronic and thermal mechanisms of the refractive index changes (RIC) in single-mode ytterbium-doped optical fibers induced by optical pulses at 980 nm and we report the general equations describing RIC evolution.In particular, the electronic RIC dynamics is shown to follow the dynamics of the population of the excited/unexcited ion states with a factor proportional to their polarizability difference (PD).The thermal contribution to the RICs induced in standard Yb-doped fibers is shown to be negligible under condition of all-fiber low-power experiments discussed in the Chapter. Electronic and thermal RIC mechanisms caused by nonlinear pump absorptionYtterbium-doped silicate glass fibers are widely used for high-power, high-beam-quality laser sources due to their impressing gain efficiency, single transverse mode quality, relaxed thermal management problems, and overall robustness to environmental disturbances.Resulting from the confined mode structure and typically long propagation distances, nonlinear phenomena such as stimulated Brillouin scattering, stimulated Raman scattering, optical Kerr effect, inter-mode four-wave mixing, soliton and supercontinuum generation are observed in fiber lasers at relatively low power due to the high field intensity in the fiber www.intechopen.comResonantly Induced Refractive Index Changes in
We report what we believe to be the first observation of coherent transient absorption changes in glasses doped with quantum dots of CdS0.12Se0.88. These observations were made at room temperature with on-resonant laser pumping. We observe a transient bleaching on the high energy side of the pump center wavelength, and a transient absorption increase on the low energy side. The transient effects last for a time about equal to the pump pulse duration, 80 fsec, after which real carrier generation masks the effects. The results can be qualitatively explained by a spectral shift in the direction predicted by the optical Stark effect. This was a pump-probe experiment where the 622-nm pump center wavelength was on the long wavelength side of the lowest energy transition in the quantum dots but still well within the absorption band. The radii of the microcrystallites ranged from 20 to 40 Å, and T2 in similar samples was estimated to be 60 fsec or less. Although the laser pulsewidth is longer than the dephasing time in the sample, we were still able to see coherent effects.
We present an experimental study of nonlinear selective reflection at a dielectric--Cs-vapor interface, in a three-level cascade-up scheme. The nonlinear reflection at the Cs D2 resonance line is monitored in the presence of resonant irradiation at 621 nm or 658 nm (6P3/2\ensuremath{\rightarrow}8D5/2,9S1/2). Experimental reflection spectra with either counterpropagating or copropagating light beams are analyzed and compared with the theoretical predictions of F. Schuller et al. [Phys. Rev. A 47, 519 (1993)]. For a blue-detuned pump and copropagating geometry an alternative type of three-level coherent reflection resonances, specifically associated with desorbing atoms, is observed. Also observed is the nonlinear reflection associated with the off-resonant, two-photon dispersion of atoms moving parallel to the window surface. Probe beam saturation is responsible for the appearance of additional resonant features in reflection spectra. Finally we analyze crossover spectral resonances which arise from nondegenerate, four-wave-mixing processes induced by the combined influence of counterpropagating pump and probe beams, and the pump beam being partially reflected by the window.
Nonlinear absorption and amplification of a probe laser beam can be controlled by adjustment of the intensity-modulation frequency and the wavelength of a pump laser beam. A demonstration of this effect in Er3+-doped fluoroindate glass is presented. The results show maximum amplification of the probe beam (∼12%) when a pump laser emitting 16 mW of power is modulated at ∼30 Hz. In the limit of low modulation frequencies, or cw pumping, induced absorption of the probe beam is the dominant nonlinear process.